I see my question was answered through the escape sequence not being needed in this case. So that's great. I still don't know how to turn off some warning I don't want to be bothered with, but that's another matter and not a real problem, at least for now. Nice to have this resource here for help and education:-)

Thanks for the extra tips and education. Always nice to know more. Plus, my original "issue" was so easy to fix:-)

FIXED ! At least for me. I just had this same problem come up, with the errors not showing while editing, and then showing up for some number of seconds after attempted build and then disappearing. M1 Mac mini, all the latest software as of today 28 Mar 2023. I discovered it was only the project I was working on. Opening another project worked fine. I started a new project and copied all the text from the files in the problem project over. All good now. I'm too new at this to know how to mark this as a solution to help others. Maybe someone has a moment to school me a little.

I did more research on this. In the Mail app in a new message, typing opt-shift-k still gives me a square instead of the Apple character. However, if I paste in the Apple character I get from typing opt-shift-k in my Notes app it shows as the Apple character in the mail message. Furthermore, after that, if I type opt-shift-k in the Mail message I get the Apple character and can do that as many times as I like. Opening a new Mail message repeats this cycle again starting with the square.... Definitely something I don't understand here. Sonoma 14.3.1, M1 Mac mini. Apple Bluetooth keyboard with keypad and Touch ID key.

After more research I figured it out. It's a font problem. I'm using Verdana in Mail and the Apple character is missing. (I'm using Verdana because it shows the capital of the i character distinct from the lower case L character. For instance, when abbreviating Artificial Intelligence to two letters it's confusing.)

This problem came back for me trying to read a simple text data file in Swift on an M4 Mac mini with the latest updates on everything... I struggled with trying different settings in the sandbox and elsewhere and moving the file into the development folder, etc... Finally I discovered I could just delete the entire App Sandbox (little trash can icon at right) and now it works :-) For now, at least....